In this paper we examine the effect of X-ray and Lyalpha photons on the intergalactic medium temperature. We calculate the photon production from a population of stars and micro-quasars in a set of cosmological hydrodynamic simulations which self-consistently follow the dark matter dynamics, radiative processes as well as star formation, black hole growth and associated feedback processes. We find that, (i) IGM heating is always dominated by X-rays unless the Lyalpha photon contribution from stars in objects with mass M<10^8 Msun becomes significantly enhanced with respect to the X-ray contribution from BHs in the same halo (which we do not directly model). (ii) Without overproducing the unresolved X-ray background, the gas temperature becomes larger than the CMB temperature, and thus an associated 21 cm signal should be expected in emission, at z<11.5. We discuss how in such a scenario the transition redshift between a 21 cm signal in absorption and in emission could be used to constraint BHs accretion and associated feedback processes.
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